I'm sure the crystals formed are either quartz, mica or even calcite. Typically, crystals are found in voids inside rocks where the rock matrix is semi permeable. Fluid flows in the minerals are all that is left when the water evaporates. Those minerals form crystals. Your rock seems at first to be sedimentary rock which formed a concretion. Soil, minerals and sand form rocks when subjected, to heat and/or pressure, such as the weight of matter compressing everything beneath it like soil layers, or limestone layers, volcanic rock like basalt.
